Northeast Flooding? Yes, the Climate Really is Different Now

Today's heavy rains and flooding in the Northeastern U.S. are a reminder of where this region's climate is headed: toward a wetter future.

Heavy rains and flooding are likely to continue in the northeastern United States over the next few days, after recent storms that caused levees to collapse, homes and vehicles to be swept away, and several reports of deaths and missing persons. It's reminiscent of flooding in 2011 that which inundated neighborhoods in New England and the mid-Atlantic states that had remained above water since being built. This is the kind of weather that makes people wonder whether we're seeing a long-term shift in climate. In this case, the answer is yes.

According to current climate research, this summer's floods are occurring 45 years into the wettest period the Northeast has seen in at least 500 years. The first scientific paper outlining this climate history was published a few months ago, by a team led by Neil Pederson, a scientist at Columbia University's Lamont-Doherty Earth Observatory. The scientists examined hundreds of narrow cores drilled from trees in eastern New York State and northern New Jersey. The cores came from 12 species that have been shown to record relative drought conditions in their tree ring structure. (The researchers also examined existing records from across the eastern United States.)

The study indicates that the climate of the East Coast has been growing wetter since the 1880s, with intermittent dry spells. And it's a steep upward curve. Pederson, who appears to be punctilious even by scientific standards, says the evidence was so surprising that he went back and rechecked the wooden core samples, his measurements, and his statistical methods multiple times before convincing himself and his colleagues that he was right. Conditions in the deep South have grown somewhat drier over the past generation, but elsewhere in the eastern third of the country, the wetting trend continues. Pederson, who lives near the Hudson River, has seen sandbags protecting sections of Nyack, N.Y., that had never before seen flooding.

Anything to the left of the zero mark is drier than the median for the past 500 years. The 20th century was much wetter than previous centuries. The last 40-year period has been an extreme outlier.

"If the Northeast continues getting wetter, larger storms, we'll continue to have sewage treatment overflows polluting the water, businesses getting flooded out, and people losing their homes," he says. On the other hand, it would be even worse if the region were to return to the conditions of previous centuries. Water managers in the Northeast faced water shortages in 2002, during a period of mild drought, and speak with shudders of a severe six-year drought in the 1960s. By historical standards, though, that wasn't a long incident.

The population centers of the Northeast have developed during a kind of Goldilocks period, when there has been plenty of water to keep the reservoirs full but not enough to cause frequent flooding. There were six significant droughts in the 1600s, including a mega-drought that lasted two decades. One early drought gained some fame in the late 1990s thanks to study led by David Stahle at the University of Arkansas, which showed that Virginia's lost Roanoke colony died out during the most extreme growing-season drought in 800 years. Thirty years later, the Jamestown colony struggled through the driest seven-year period over that time span. The dry times may not return any time soon, but for now that idea relies more on wishful thinking than on science.

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